Fastening means for two-piece core bit

ABSTRACT

The core bit is removably secured to a power tool shank by threaded fastening means consisting of double sets of helical threads. The threads have an arcuate cross-sectional shape. The helical angle of the threads is at least five degrees.

United States Patent Meyer FASTENING MEANS FOR TWO-PIECE CORE BIT Apr.8, 1975 3,388,935 6/1968 Hjalsten et al .6 403/343 PrimaryExaminer-Werner H. Schroeder Assistant Examiner- Conrad L. Berman [57]ABSTRACT The core bit is rcmovably secured to a power tool shank bythreaded fastening means consisting of double sets of helical threads.The threads have an arcuate cross-sectional shape. The helical angle ofthe threads is at least five degrees.

2 Claims, 3 Drawing Figures FASTENING MEANS FOR TWO-PIECE CORE BITBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to core bits, and more particularly to fastening meansfor socalled two-piece core bits. By the term core bits, reference ismade to generally hollow cylindrical devices having an annular cuttingformation at one end thereof for drilling holes of substantial diameter,say, on the order of at least two or three inches, in materials, such asconcrete and the like. These core bits are provided in various sizes,i.e., with different diameters, to permit the drilling or forming ofdifferent sizes of holes in the material being engaged by the core bit.Preferably, these core bits of various sizes are adapted for detachableengagement with a single tool shank of a portable power tool.

2. The Prior Art It is known in the prior art to provide a threadedconnection between the power tool shank and the various core bits to bedriven by such tool shank. As is known to those skilled in the art, corebits of the type under construction are subjected to vibrational andtorque forces of very high magnitude. Such forces tend to cause the corebit to become locked to the tool shank, making removal of the core bitfrom the shank extremely difficult, if not impossible, without theinfliction of damage to the core bit.

SUMMARY AND OBJECTIVES OF THE INVENTION The present invention relates tonew and improved fastening means for securing a core bit to a power toolshank, such fastening means permitting ready attachment of the core bitto the shank as well as ready and easy detachment of the core bit fromthe shank.

A primary object of the present invention is the pro vision of a new andimproved threaded fastening means for securing a core bit to a powertool shank.

Another object of the present invention is the provision of threadedfastening means for securing a core bit to a tool shank, such fasteningmeans including a first set of double threads on the shank and a secondset of complementary double threads on the core bit.

Still another object of the present invention is the provisison ofthreaded fastening means according to the foregoing objects, whereinsuch threads have a hellcal angle of at least five degrees.

Another object of the present invention is the provision of threadedfastening means according to the foregoing objects wherein such threadshave an arcuate cross-sectional shape.

These and other objects and advantages of the invention will becomeapparent from the following specification disclosing a preferredembodiment illustrated in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of a powertool shank embodying the present invention;

F 10. Z is a partial elevational and partial longitudinal section of acore bit embodying the present invention; and

FIG. 3 is an end elevation of the core bit as seen looking at thecutting face thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT A tool shank, generallydesignated 10, includes a hexagonal formation 11 at one end thereof. Itwill be understood that such hexagonal formation is adapted to bereceived within the rotary driving member of a power tool so as totransfer rotational movement from such power tool to the tool shank. Ofcourse, other formations, such as a spline drive, may be provided inlieu of the hexagonal formation.

The other end of the tool shank is defined by a threaded formation,generally designated 12, which formation is separated from the hexagonalformation II by a cylindrical shank portion 14. The formation 12 isdefined by a pair of helical threads l5, 16; in other words, theformation 12 consists of a set of double threads. The pitch of each ofthe threads 15 and I6 is, of course, the same; the pitch of the thread15 is designated in FIG. I as P. It should be noted that thecrosssectional shape of the threads is smooth and arcuate in form asopposed to being V-shaped. As noted in FIGS. 1 and 2, the crests of thethreads adjoin the roots of the adjacent threads in an uninterruptedmanner. In other words, the threads do not have sharp edges.

The helical angle of the threads is designated in FIG. I and A. Thisangle is at least five degrees; preferably this angle will be in therange of eight to twelve degrees. In the preferred form of theinvention, the minimum helical extent for either one of the threads l5,16 is in the order of 720". It should also be mentioned that the threads15 and 16 are not deeply formed into the body portion of the shank 10.In the embodiment shown for purposes of illustration, the minor diameterof the threads is at least of the major diameter of the threads.

A core bit, generally designated 18, is seen to include a hollowcylindrical body portion 19 having an integral helical rib 20 over aportion of the outer surface thereof. The body portion 19 includes anannular sur face 21 at one end thereof, which surface includes aplurality of notches or recesses 22 in which carbide drilling tips 23are mounted. Although only four of such tips are illustrated in FIGS. 2and 3, it will be understood that a greater number of tips are providedin actual practice; for example, a core bit having an outside diameterof three inches may be provided with fourteen drilling tips. Of course,the diameter of the body portion 19 will vary to provide core bits ofdifferent sizes for drilling holes of various diameters.

The body portion 19 of the core bit includes a reduced-in-diameterportion 25 at the end thereof remote from the annular cutting face ofthe bit. When the core bit has a body portion diameter smaller than thediameter of the core bit illustrated, the portion 25 may have an outsidediameter the same as the outside diameter of the portion 19. Thereduced-in-diameter portion 25 includes a central portion 26communicating with the interior of the body portion 19 of the core bit.The bore 26 includes a threaded formation, generally designated 28,complementary with the threaded formation 12 on the tool shank forthreading interengagement therewith. Since the threaded formation 28 iscomplementary in all respects with the threaded form ation 12, theformer need not be described in detail. It is sufficient to say that thethreaded formation 28 in cludes a double set of helical threads 29, 30having the same cross-sectional shape, helical angle and helical extentas the complementary threads l5, 16.

It is understood, of course, that the core bit 18 is detachably securedto the tool shank by interengagement of the threaded formation 12 and28. During tightening of the core bit to the tool shank. relativerotation between these two members is preferably stopped by abuttingengagement of an annular surface 26a on the core bit with an annularflange 14a on the tool shank.

It has been found. by the use of tools embodying the present invention.that it is virtually impossible for the core bit to become locked to thetool shank. notwithstanding the fact that the core bit is subjected torotational and vibrational forces of very high magnitude. Locking of thecore bit to the tool shank is prevented to a large degree by theprovision of the double threads which, in effect, establishes a pitchdouble the pitch that would he achieved by using a single thread. Thisincrease in pitch lessens the tendency of the threads to lock up witheach other.

Another feature contributing to the prevention of this lockup is therather low helical angle A for the threads maintaining such angle atleast five degrees prevents the threads from interlocking with eachother. The provision of rounded or arcuate threads eliminates sharpsurfaces or edges, thereby reducing stress concentrations and theirconsequent fatique breaks or fractures.

Since the tool shank l0 and the core bit 18 do not become lockedtogether, these parts may tend to separate prematurely and inadvertentlyupon their removal from the work as the operator will normally rotatethe shank in the opposite direction to facilitate such removal. In suchan event. the tool shank will tend to rotate relative to the core bit asthe latter will resist rotation to some degree due to its frictionalengagement with the work. To prevent such unintended separation. thetool shank 18 is preferably provided with an annular recess 32 mountinga high temperature resistent. flexible O-ring 33. When the tool shank isthreaded with the core bit 18, this O-ring will be received within thebore 26 and squeezed into engagement with the wall of the latter TheO-ring 33 provides sufficient frictional engagement with the tool skankand the core bit to prevent their inadvertent separation during removalfrom the work. However, the frictional forces developed by the O-ringare not so great as to prevent the ready and easy manual separation ofthe tool shank and core bit.

I claim:

1. A drilling bit assembly including a cylindrical core bit and a powertool shank element detachably secured together by fastening means, saidfastening means being defined by a first set of double threads on saidshank element and a second set of double threads on said core bit, saidfirst and second sets of threads being of complimentary external andinternal configuration for threading interengagement thereby to fastensaid core bit to said shank element, the diameter of the one of saidsets of threads, which is of external configuration, being slightly lessthan the diameter of the other of said sets of threads, which is ofinternal configura' tion, thereby to facilitate their interengagement,said first and second sets of threads being otherwise identicaloppositehand versions of each other in all respects, said first andsecond sets of threads being in substantial co-extensive surfaceengagement with each other throughout substantially all of theirrespective helical extents thereby securely but detachably securing thecore bit to the shank element. the cross-sectional shape of said threadsbeing smooth and arcuate with the arcuate crests of the male threadsadjoining the arcuate roots of the female threads in an uninterruptedmanner, the portions of the surfaces of said threads between the zenithof the crests thereof and the nadir of the roots thereof being containedin planes defining a substantial angle with the co-axial longitudinalcentral axes of said bit and said shank. the minor diameter of saidthreads being at least percent of the major diameter thereof. the helixdefined by each of said threads having an extent of at least 720degrees, the helical angle of each of said first and second sets ofthreads being at least five degrees.

2. The improvement according to claim 1 wherein said first set ofthreads is formed externally of said shank at one end thereof andwherein said second set of threads is formed internally on said core bitat one end thereof.

1. A drilling bit assembly including a cylindrical core bit and a powertool shank element detachably secured together by fastening means, saidfastening means being defined by a first set of double threads on saidshank element and a second set of double threads on said core bit, saidfirst and second sets of threads being of complimentary external andinternal configuration for threading interengagement thereby to fastensaid core bit to said shank element, the diameter of the one of saidsets of threads, which is of external configuration, being slightly lessthan the diameter of the other of said sets of threads, which is ofinternal configuration, thereby to facilitate their interengagement,said first and second sets of threads being otherwise identicaloppositehand versions of each other in all respects, said first andsecond sets of threads being in substantial co-extensive surfaceengagement with each other throughout substantially all of theirrespective helical extents thereby securely but detachably securing thecore bit to the shank element, the cross-sectional shape of said threadsbeing smooth and arcuate with the arcuate crests of the male threadsadjoining the arcuate roots of the female threads in an uninterruptedmanner, the portions of the surfaces of said threads between the zenithof the crests thereof and the nadir of the roots thereof being containedin planes defining a substantial angle with the co-axial longitudinalcentral axes of said bit and said shank, the minor diameter of saidthreads being at least 80 percent of the major diameter thereof, thehelix defined by each of said threads having an extent of at least 720degrees, the helical angle of each of said first and second sets ofthreads being at least five degrees.
 2. The improvement according toclaim 1 wherein said first set of threads is formed externally of saidshank at one end thereof and wherein said second set of threads isformed internally on said core bit at one end thereof.